There are countless applications that require knowledge of the topography of the surface of an object and thus determination of its microscopic shape. Such analysis may, for example, form part of a quality control process.
The introduction of new applications for materials and new manufacturing processes is resulting in a systematic increase in the market demand for processes for the inspection of surfaces and for the development of optical profilometers.
In this connection, there are at present two technologies based on image formation that have competed fiercely in recent years to dominate the optical surface metrology market.
Both the confocal technique and the interferometric technique are able to measure surface topography precisely and reliably on the micrometric and nanometric scales.
Nevertheless, as explained below, the measurement principles involved in the confocal technique and the interferometric technique are very different, and consequently the capacities of the two techniques are more complementary than coincident.
Confocal Profilometry.
Confocal profilometers allow height measurement of surfaces with a wide range of textures (from very rough surfaces to very smooth ones) by scanning the sample vertically in steps so that each point on the surface passes through the focus. The height of the surface at each point is established by detecting the position of the maximum value of the axial response. Since only one or a very small number of points on the surface are illuminated simultaneously, an in-plane scan must be carried out in order to build up the axial response, i.e. the confocal image, in each vertical step for all the points falling within the field of view of the lens used.
Examples of this type of profilometer are described, for example, in European patent EP0,485,803, which refers to an optical device that uses the path of a confocal beam with an illumination array and a detector array. The illumination array is imaged in a focal plane located on or in the vicinity of the surface of the object. The radiation reflected in the focal plane is sent directly, by means of a beam splitter, onto the receiver surface of a CCD device. The imaging of the illumination array on the receiver surface is performed so that the light-sensitive regions of the receiver act as confocal diaphragms. The signals from the elements of the detector that receive only light scattered outside the focal place are not taken, or are taken separately, into account in the evaluation.
U.S. Pat. No. 5,239,178 discloses a similar optical device in which an illumination grid and a detector grid are provided in a confocal arrangement in relation to an object. The illumination grid may have a variable grid size formed, for example, of a grid of LEDs.
Interferometric Profilometry.
In this case a beam of light passes through a beam splitter. One part of the beam is sent to the surface of the sample and the other part is sent to a reference mirror. The light reflected from these surfaces is recombined and forms a pattern of interference fringes.
Interferometric profilometry makes use of phase shift interferometry (PSI) that allows measurement of the topography of very smooth surfaces with subnanometric resolution. The sample, which must be placed at the focus, is scanned vertically in steps that are a highly precise fraction of the wavelength. Profiling algorithms produce a phase map of the surface, which is converted into the corresponding height map by means of the suitable unwrapping procedure.
Interferometric profilometers also use vertical scanning interferometry (VSI) with white light to measure the topography of smooth or moderately rough surfaces. Maximum contrast of the interference fringes occurs at the best position of the focus for each point on the surface of the sample. The sample is scanned vertically in steps so that each point on the surface passes through the focus. The height of the surface at each point is obtained by detecting the position of the maximum of the envelope of the interferogram.
Examples of this type of profilometer are described, for example, in U.S. Pat. No. 5,563,706, referring to an interferometric surface profilometer. The light reflected from a reference surface and a sample surface is sent to an imaging optical system through a beam splitter and interference fringe formed from light reflected from the both surfaces is observed with a detection optical system. An alignment optical member is disposed in an optical path between the imaging optical system and an image plane of the interference fringe, so that the rear focal point of the alignment optical element is located at the image plane.
For example, U.S. Pat. No. 6,665,075 describes an imaging system using a phase shift interferometer (PSI) and further including a transmitter, a beam splitter, a phase inverter and a receiver. The transmitter transmits a signal pulse that is split into a measurement pulse and a reference pulse. The measurement pulse is applied to a sample and a relative phase shift is introduced between the two pulses by the phase inverter, which are recombined to form a combined pulse that is detected by the receiver.
U.S. Pat. No. 6,636,317 provides that the beam splitter is arranged with a little inclination from the vertical to the incident light beam. In the optical interferometer described, the incident light beam is branched into two optical paths which cross at right angles by means of said beam splitter. In each optical path, the reflection light is totally reflected by a first reflection unit while transmission light is totally reflected by a second reflection unit. The reflection lights by both reflection units are wave-combined again by the beam splitter and received by a light receiver.
The above mentioned PSI and VSI interferometers can carry out very fast measurements on the micrometric and nanometric scales. In addition, there is no intrinsic limitation in the vertical measurement range with the VSI technique. Nevertheless, both techniques have the drawback that they cannot easily deal with highly inclined smooth surfaces or with structured samples containing dissimilar materials.
PSI devices allow users to carry out measurements of shape and texture even at scales lower than 0.1 nm. Nevertheless, they have the drawback of an extremely limited vertical measurement range.
Confocal profilometers based upon image formation as described above provide solutions to many of the difficulties involved in the interferometry technique and can even provide the best lateral resolution possible with an optical system. Nevertheless, they have a limited vertical resolution imposed by their numerical aperture value and do not allow attainment of repeatabilities on the order of 0.1 nm.
Confocal profilometers may be used with lenses with high magnification and numerical aperture, up to 150× and 0.95, respectively. On the other hand, the magnification possible with PSI and VSI devices is limited in practice to 50×. Higher magnification can be attained through the use of other types of lenses, by they are impractical and extremely expensive.
It is therefore desirable to have a profilometer that can provide representations of smooth or relatively rough surfaces with a certain degree of inclination to determine their shape, texture, etc. in samples of dissimilar materials with resolution on a subnanometric scale.